1. Field of the Invention
The present invention relates to a radiation-curable polymerizable composition. More specifically, present invention relates to an ink composition suitable for use in inkjet recording, an inkjet recording method and printed materials produced using the method. Furthermore, the invention relates to a planographic printing plate obtained using the ink composition and a method for forming the planographic printing plate.
2. Description of the Related Art
There are various image recording methods for forming images on a recording material, such as paper, based on image data signals, such as electrophotographic methods, sublimation and fusion thermal transfer methods, and inkjet methods. Among these, inkjet methods can be implemented with inexpensive apparatus, and allow efficient use of ink since jets of ink are directed only at areas required for image formation, and form images directly on a recording material, resulting in reduced running costs. In addition, inkjet methods permit printing with a low noise level, and thus they are excellent image recording methods.
In inkjet methods, although printing can be performed, in addition to on paper, on a recording material having no water-absorbing properties, such as a plastic sheet or a metal sheet, there are significant problems in speeding up printing and enhancing image quality. More specifically, in the inkjet method, the time required for drying and curing of ink droplets after printing greatly affects the productivity in printing, as well as the sharpness of printed images.
One of the conventionally-known inkjet methods is a recording method using inkjet recording ink curable by exposure to radiation. According to this method, ink droplets are cured by exposure to radiation immediately after ink jetting or after a certain lapse of time from ink jetting, and thereby productivity in printing can be improved and sharp images can be formed.
By achieving an increase in sensitivity of inkjet recording ink curable by exposure to radiation such as ultraviolet rays, the inkjet recording ink can have high curability when irradiated; as a result, there arise many benefits including a productivity increase in inkjet recording, a reduction in power consumption, an increase in the longevity of a radiation generator through reduction of a load thereon, and prevention of evaporation of low-molecular substances caused by insufficient curing. In addition, the increase in sensitivity enhances the strength of images formed by ink for inkjet recording in particular, and, when high-sensitivity inkjet recording ink is applied to making of a planographic printing plate in particular, the cured strength of image areas is increased and high printing durability can be attained.
In recent years, the inkjet method wherein curing by radiation such as ultraviolet rays takes place has been receiving attention due to it having a relatively weak odor, quick-drying properties and because it allows recording on recording materials having no ink absorbency, and consequently ultraviolet cured inkjet ink compositions utilizing radical polymerization have come to be known. Additionally, cationic polymerization ink compositions have been put forth for the purpose of promoting adhesion to recording materials (See, e.g., Japanese Patent Application Laid-Open (JP-A) No. 9-183928). However, these ink compositions have insufficient stability during storage owing to a reaction based on acids that occurs over time, which constitutes an obstacle to practical utilization. As an attempt to improve storage stability, the technique of adding a basic compound thereto was therefore proposed (See JP-A No. 2003-341217). However, a new problem of a drop in the curing sensitivity of the ink arose, because the basic compound impaired the function of an acid produced by exposure. As a solution to this problem, an ink composition containing a photostable compound was proposed (JP-A No. 2004-238456), but an increase in sensitivity together with storage stability is yet to be sufficiently achieved.